1. Technical Field
The present invention relates to a load control unit for controlling a load correspondingly to a fixed input without using a constant voltage source.
2. Background Art
Usually, as a load control unit for controlling a load for a vehicle such as a lamp mounted on a vehicle in accordance with a pulse width modulation (PWM), a load control unit has been known that meets two input patterns of a fixed input and a pulse input from a control unit such as an electronic control unit (ECU) mounted on a vehicle or a control unit of a relay integration.
In such a kind of the load control unit, a driving control signal is generated whose level is changed by a prescribed frequency (period) and duty relative to the fixed input, or a driving control signal is generated whose level is changed by the same frequency and duty as those of the pulse input to control the load for the vehicle to be driven by the driving control signal.
As described above, the load control unit is made to meet the two input patterns, because the load control unit is allowed to have general-purpose characteristics that can widely meet many kinds of the ECUs or the relay integration different depending on the grades of the vehicles.
Ordinarily, in order to allow the load control unit to meet the two input patterns, a circuit for the fixed input whose circuit structure is complicated is firstly designed, and then, when the load control unit is made to meet the pulse input, the input and output of the load control unit are short-circuited in the circuit for the fixed input.
Specifically, a below-described method is proposed. That is, in this method, a separate IC for the pulse input is formed by changing a wiring pattern in an IC of the load control unit whose circuit is designed for the fixed input.
However, in this method, since a production process for changing the wiring pattern by using a photolithography technique and an etching technique is necessary, the production process is complicated and the number of photo-masks is increased. Thus, a problem arises that a production cost is increased.
Further, since the two kinds of ICs for the fixed input and the pulse input cannot be discriminated from each other at a glance, a problem arises that it is almost misleading to discriminate between both the ICs in an inspection process after the production process. Further, since the management of parts of the two kinds of ICs is troublesome, a cost for managing the parts is disadvantageously increased.
Further, a method is also proposed that a switch for short-circuiting the input and output of the load control unit is added to an inner part of the IC of the load control unit whose circuit is designed for the fixed input, and the switch is operated from an external part of the IC to change the IC to the IC for the pulse input.
According to this method, since the two input patterns can be met by the one IC, the respective problems in the above-described former method for forming another IC for the pulse input can be avoided. However, in the latter method for operating the switch for short-circuiting the input and output of the load control unit from the external part of the IC to change the IC to the IC for the pulse input, an operating terminal for operating the switch from the external part needs to be provided in the IC of the load control unit. In addition thereto, when the switch is operated, an external attaching element such as a resistor needs to be attached to the operating terminal. Therefore, since the number of terminals of the IC of the load control unit is increased and the externally attaching element is necessary, a problem arises that the mounting characteristics of the IC are deteriorated and the cost of the load control unit is increased.
As a load control unit that solves these problems, Patent Document 1 proposes a load control unit as shown in FIG. 4 that can meet two input patterns of a fixed input and a pulse input.
Namely, FIG. 4 shows a schematic structure of the load control unit in the Patent Document 1. In this load control unit, a voltage of a capacitor C1 is compared with a reference voltage Vt1 by a comparator CP1 on the basis of a current mirror circuit including resistors R6, R7 and R9 and transistors Q10 and Q11 connected to a constant voltage power source (Vc) 11 of the load control unit 10 and a constant current obtained by a resistor R1, the charging/discharging operations of the capacitor C1 are switched to generate a triangle wave, and this triangle wave is compared with a divided voltage Vk inputted to a plus input terminal of a comparator CP3 to obtain a PWM pulse.
In the drawing, there are provided an OR gate 12, a driving circuit 13, a load driving element 14 (FET) of Nch, a load 15, an ECU 20, and a battery 30 mounted on a vehicle.
Now, an operation of the load control unit in FIG. 4 will be described by referring to a timing chart shown in FIG. 5. One terminal of the capacitor C1 is connected to a charging/discharging circuit and the other terminal is connected to a GND terminal.
A circuit for generating the triangle wave includes transistors Q3 to Q11, resistors R1 to R10, the capacitor C1 and the comparator CP1. A constant current serving as a reference for charging/discharging the capacitor C1 is generated by a current mirror circuit composed of the transistors Q3 to Q5, and a current I5 supplied from the transistor Q5 serves to charge the capacitor C1.
A current I1 supplied from the transistor Q3 and a current I4 supplied from the transistor Q4 are values respectively equal to the current I5 supplied from the transistor Q5 as shown by a below-described expression (1).I1=I4=I5  (1)
The current I1 is set by the resistor R1 and expressed by a below-described expression (2).I1=(Vc−Vbe3)/R1  (2)
Here, Vbec designates a voltage between a base and an emitter of the transistor Q3.
Further, a current I6 expressed by a below-described expression (3) for discharging the capacitor C1 is generated by a current mirror circuit composed of the transistors Q6, Q8 and Q9.I6=2*I4=2*I1  (3)
To the base of the transistor Q6, the transistor Q7 for a switch is connected. When the transistor Q7 is turned on, 16 is not supplied. Further, the reference voltage Vt1 is generated by the resistors R4, R5 and R6 connected in series to the Vc from the constant voltage source 11 and the transistor Q10. Under a state that the transistor Q10 is turned off, the reference voltage Vt1 becomes the upper limit voltage Vb of the triangle wave and is expressed by a below-described expression (4).Vb=Vc*R5/(R4+R5)  (4)
Under a state that the transistor Q10 is turned on, the reference voltage Vt1 is a divided voltage of a combined resistance of the resistors R5 and R6 and the resistor 4 by neglecting the saturation voltage of the transistor Q10. The lower limit voltage Va of the triangle wave is expressed by a below-described expression (5).Va=Vc*(R5*R6)(R4*R5+R4*R6+R5*R6)  (5)
When the voltage VC1 of the capacitor C1 is lower than the reference voltage Vt1 at a certain moment, the output of the comparator CP1 is Lo to turn off the transistors Q10 and Q11. When the transistor Q10 is turned off, the reference voltage Vt1 becomes a value of the upper limit voltage Vb of the triangle wave. When the transistor Q11 is turned off, since a current is supplied to the base of the transistor Q7 from the resistor R10, the transistor Q7 is turned on to turn off the transistors Q6, Q8 and Q9 and 16 is not supplied.
As a result, the capacitor C1 is charged by the I5 so that the voltage VC1 rises. When the voltage VC1 of the capacitor C1 even slightly exceeds the upper limit voltage Vb of the triangle wave, the output of the comparator CP1 becomes Hi to turn on the transistors Q10 and Q11. At this time, the transistor Q10 is turned on so that the reference voltage Vt1 falls to a value of the lower limit voltage Va of the triangle wave. When the transistor Q11 is turned on, since the transistor Q7 is off, the transistors Q6, Q8 and Q9 are turned on to supply the I6.
Since the I6 is two times as large as the I5, when the I5 is subtracted from the I6, the capacitor C1 is discharged by a current value of the I5. When the voltage VC1 of the capacitor C1 falls and slightly falls below the lower limit voltage Va of the triangle wave, the output of the comparator CP1 is inverted to Lo. In such a way, the triangle wave is obtained.
A period T of the triangle wave is expressed by a below-described expression (6).
                                                        T              =                            ⁢                              2                *                                  (                                      Vb                    -                    Va                                    )                                *                C                ⁢                                                                  ⁢                                  1                  /                  I                                ⁢                                                                  ⁢                1                                                                                        =                            ⁢                              [                                  2                  *                  Vc                  *                                      {                                          (                                                                        R                          ⁢                                                                                                          ⁢                                                      5                            /                                                          (                                                                                                R                                  ⁢                                                                                                                                          ⁢                                  4                                                                +                                                                  R                                  ⁢                                                                                                                                          ⁢                                  5                                                                                            )                                                                                                      -                                                                                                                                                                                                    ⁢                                                      (                                          R                      ⁢                                                                                          ⁢                      5                      *                      R                      ⁢                                                                                          ⁢                      6                                        )                                    /                                      (                                                                  R                        ⁢                                                                                                  ⁢                        4                        *                        R                        ⁢                                                                                                  ⁢                        5                                            +                                              R                        ⁢                                                                                                  ⁢                        4                        *                        R                        ⁢                                                                                                  ⁢                        6                                            +                                              R                        ⁢                                                                                                  ⁢                        5                        *                        R                        ⁢                                                                                                  ⁢                        6                                                              )                                                  }                            *                                                                                                          ⁢                                  C                  ⁢                                                                          ⁢                  1                                ]                            /                              [                                                                            (                                              Vc                        -                                                  Vbe                          ⁢                                                                                                          ⁢                          3                                                                    )                                        /                    R                                    ⁢                                                                          ⁢                  1                                ]                                                                        (        6        )            
A circuit for generating the PWM pulse includes resistors R11 and R12 and the comparator CP3. The divided voltage Vk is obtained by dividing the voltage by the resistors R11 and R12 and is expressed by a below-described expression.Vk=Vc*R12/(R11+R12)  (7)
When the voltage VC1 of the capacitor C1 is higher than the divided voltage Vk, the output of the comparator CP3 is Lo. When the voltage VC1 of the capacitor C1 is lower than the divided voltage Vk, the output of the comparator CP3 is Hi.
Further, the output of the comparator CP3 is inputted to the driving circuit 13 via the OR gate 12. In the driving circuit 13, the input is inverted and outputted to the gate of the load driving element (FET) 14. When a control input is Hi, a gate voltage of the load driving circuit (FET) 14 is fixed to Lo so that the load driving element (FET) 14 is turned off and a load current is zero.
When the control input from the ECU 20 is Lo, the pulse of the comparator CP3 is made to be effective by the OR gate 12. When the output of the comparator CP3 is Lo, the output of the driving circuit 13 is Hi to turn on the load driving element (FET) 14 and supply the load current IL.
Here, a voltage drop arises in a wiring resistance RW by the load current IL. However, even when VIN as a voltage from the battery 30 mounted on a vehicle varies, since the Vc is maintained to a constant level by the constant voltage source 11, the accuracy of a frequency and a duty ratio can be maintained without especially considering the voltage characteristics of the circuit.
[Patent Document 1] JP-A-2001-148294
In the load control unit disclosed in the above-described Patent Document 1, even when the VIN as the voltage from the battery 30 mounted on a vehicle varies, since the Vc is maintained to the constant level by the constant voltage source 11, the accuracy of the frequency and the duty ratio are maintained without receiving an influence due to the variation of the VIN. However, there is a problem that the scale of a circuit of the constant voltage source 11 is large and the cost thereof is high.
In order to solve such a problem, a measure may be considered that the constant voltage source 11 is saved and other means is used so as not to lower the accuracy of a PWM output. Now, this method will be studied as described below.
In the case of a 12 V type power source of a motor vehicle, the VIN as the voltage from the battery 30 mounted on a vehicle that is a power source varies to 9 to 15 V or so. The period T of the triangle wave in this case is expressed by a below-described expression (8) by replacing Vc by VIN in the above-described expression (6).T=[2*VIN*{(R5/(R4+R5)−(R5*R6)/(R4*R5+R4*R6+R5*R6)}*C1][(VIN−Vbe3)/R1]  (8)
Here, since the Vbe3 as the voltage between the base and the emitter of the transistor Q3 is about 0.6V, assuming that the VIN−Vbe3 is substantially equal to the VIN, the period T of the triangle wave is expressed by a below-described expression (9).
                                                        T              =                            ⁢                              [                                  2                  *                  VIN                  *                                      {                                          (                                                                        R                          ⁢                                                                                                          ⁢                                                      5                            /                                                          (                                                                                                R                                  ⁢                                                                                                                                          ⁢                                  4                                                                +                                                                  R                                  ⁢                                                                                                                                          ⁢                                  5                                                                                            )                                                                                                      -                                                                              (                                                          R                              ⁢                                                                                                                          ⁢                              5                              *                              R                              ⁢                                                                                                                          ⁢                              6                                                        )                                                    /                                                      (                                                                                          R                                ⁢                                                                                                                                  ⁢                                4                                *                                R                                ⁢                                                                                                                                  ⁢                                5                                                            +                                                              R                                ⁢                                                                                                                                  ⁢                                4                                *                                                                                                                                                                                                                                                                                                                                                                                  ⁢                                                                        R                          ⁢                                                                                                          ⁢                          6                                                +                                                  R                          ⁢                                                                                                          ⁢                          5                          *                          R                          ⁢                                                                                                          ⁢                          6                                                                    )                                        }                                    *                  C                  ⁢                                                                          ⁢                  1                                ]                            /                              [                                                      VIN                    /                    R                                    ⁢                                                                          ⁢                  1                                ]                                                                                        =                            ⁢                              2                *                                  {                                      (                                                                  R                        ⁢                                                                                                  ⁢                                                  5                          /                                                      (                                                                                          R                                ⁢                                                                                                                                  ⁢                                4                                                            +                                                              R                                ⁢                                                                                                                                  ⁢                                5                                                                                      )                                                                                              -                                                                        (                                                      R                            ⁢                                                                                                                  ⁢                            5                            *                            R                            ⁢                                                                                                                  ⁢                            6                                                    )                                                /                                                  (                                                                                    R                              ⁢                                                                                                                          ⁢                              4                              *                              R                              ⁢                                                                                                                          ⁢                              5                                                        +                                                          R                              ⁢                                                                                                                          ⁢                              4                              *                              R                              ⁢                                                                                                                          ⁢                              6                                                        +                                                                                                                                                                                                                                                                          ⁢                                      R                    ⁢                                                                                  ⁢                    5                    *                    R                    ⁢                                                                                  ⁢                    6                                    )                                }                            *              C              ⁢                                                          ⁢              1              *              R              ⁢                                                          ⁢              1                                                          (        9        )            
Here, according to the expression (9), it is understood that T is determined only by C and R and does not depend on the VIN as the voltage from the battery 30 mounted on the vehicle. The duty ratio D is represented by D=ton/T. However, D is expressed by a below-described expression (10) in accordance with a graphical relation in FIG. 5.D=(Vb−Vk)/(Vb−Va)  (10)
Further, D is expressed by a below-described expression (11) in accordance with the above-described expressions (4), (5) and (7).
                                                        D              =                            ⁢                              [                                  VIN                  *                                                            {                                                                        (                                                      R                            ⁢                                                                                                                  ⁢                                                          5                              /                                                              (                                                                                                      R                                    ⁢                                                                                                                                                  ⁢                                    4                                                                    +                                                                      R                                    ⁢                                                                                                                                                  ⁢                                    5                                                                                                  )                                                                                                              }                                                -                                                  VIN                          *                                                      {                                                                                          R                                ⁢                                                                                                                                  ⁢                                                                  12                                  /                                  R                                                                ⁢                                                                                                                                  ⁢                                11                                                            +                                                              R                                ⁢                                                                                                                                  ⁢                                12                                                                                      )                                                                                              ]                                        /                                                                                                                                        ⁢                              [                                  VIN                  *                                      {                                                                  (                                                  R                          ⁢                                                                                                          ⁢                                                      5                            /                                                          (                                                                                                R                                  ⁢                                                                                                                                          ⁢                                  4                                                                +                                                                  R                                  ⁢                                                                                                                                          ⁢                                  5                                                                                            )                                                                                                      }                                            -                                              VIN                        *                                                  {                                                                                    (                                                              R                                ⁢                                                                                                                                  ⁢                                5                                *                                R                                ⁢                                                                                                                                  ⁢                                6                                                            )                                                        /                                                          (                                                                                                R                                  ⁢                                                                                                                                          ⁢                                  4                                  *                                  R                                  ⁢                                                                                                                                          ⁢                                  5                                                                +                                                                                                                                                                                                                                                                                                                ⁢                                                      R                    ⁢                                                                                  ⁢                    4                    *                    R                    ⁢                                                                                  ⁢                    6                                    +                                      R                    ⁢                                                                                  ⁢                    5                    *                    R                    ⁢                                                                                  ⁢                    6                                                  )                            }                                                                          =                            ⁢                              [                                                      (                                                                  R                        ⁢                                                                                                  ⁢                                                  5                          /                                                      (                                                                                          R                                ⁢                                                                                                                                  ⁢                                4                                                            +                                                              R                                ⁢                                                                                                                                  ⁢                                5                                                                                      )                                                                                              -                                              R                        ⁢                                                                                                  ⁢                                                  12                          /                                                      (                                                                                          R                                ⁢                                                                                                                                  ⁢                                11                                                            +                                                              R                                ⁢                                                                                                                                  ⁢                                12                                                                                      )                                                                                                                ]                                    /                                      [                                          R                      ⁢                                                                                          ⁢                                              5                        /                                                  (                                                                                    R                              ⁢                                                                                                                          ⁢                              4                                                        +                                                                                                                                                                                                                                                                          ⁢                                      R                    ⁢                                                                                  ⁢                    5                                    )                                -                                                      (                                          R                      ⁢                                                                                          ⁢                      5                      *                      R                      ⁢                                                                                          ⁢                      6                                        )                                    /                                      (                                                                  R                        ⁢                                                                                                  ⁢                        4                        *                        R                        ⁢                                                                                                  ⁢                        5                                            +                                              R                        ⁢                                                                                                  ⁢                        4                        *                        R                        ⁢                                                                                                  ⁢                        6                                            +                                              R                        ⁢                                                                                                  ⁢                        5                        *                        R                        ⁢                                                                                                  ⁢                        6                                                              )                                                              ]                                                          (        11        )            
Here, as understood from the expression (11), D is determined only by resistance values and is not affected due to the variation of the VIN.
However, in the actual load control unit 10, when the wiring resistance RW exists with a value that cannot be neglected, an operation as shown in FIG. 6 is exhibited.
That is, when the voltage VC1 of the capacitor C1 is higher than the divided voltage Vk to turn on the load driving element (FET) 14, the load current IL is supplied and the voltage drop by the wiring resistance RW arises. Since the divided voltage Vk is a resistance divided value of a source voltage, as a power source is more lowered, the divided voltage Vk gradually falls.
At this time, the reference voltage Vt1 of the triangle wave is likewise lowered. On the other hand, the voltage VC1 of the capacitor C1 is determined by an electric charge of the capacitor C1, and accordingly, even when the VIN is lowered, since the electric charge is not outputted nor inputted, the voltage VC1 is not lowered.
Since a current for charging/discharging the capacitor C1 is generated on the basis of the VIN, when the VIN is lowered, the current is decreased correspondingly thereto to change the inclination of the voltage VC1 of the capacitor C1. Then, when the voltage VC1 of the capacitor C1 is lower than the divided voltage Vk, since the load driving element (FET) 14 is turned off to stop the load current IL, the voltage of the VIN is reset and the divided voltage Vk is also reset.
However, since the discharging current of the capacitor C1 is changed, the inclination of the voltage VC1 of the capacitor C1 is merely changed. In this case, since the reference voltage Vt1, the divided voltage Vk and the voltage VC1 respectively receive the influence of the variation of the voltage due to the wiring resistance RW attended by the load current IL and are be individually varied, the above-described expressions (9) and (11) are not established. Thus, the period T of the triangle wave and the duty ratio D are varied.
As described above, in the variation of the VIN longer and slower than the period of a PWM, the period T of the triangle wave and the duty ratio D are not varied, however, when the VIN is varied by the wiring resistance RW during the period of the PWM, the period T of the triangle wave and the duty ratio D are varied. As the wiring resistance RW is larger, an amount of variation of them is also more increased.
As described above, in order to solve the problem that the constant voltage source 11 is large in its scale of a circuit and high in its cost, when the constant voltage source 11 is simply omitted, if the influence of the wiring resistance RW exists, the accuracy of a PWM output is deteriorated. Accordingly, a load control unit 10 has been desired to be developed in which even when the constant voltage source is omitted and the influence due to the variation of the VIN as the voltage from the battery 30 mounted on a vehicle or the influence due to the wiring resistance RW exists, the accuracy of the PWM output is not deteriorated.